The present invention relates to a printing form for rotary screen printing, comprising a thin-walled hollow cylinder made from fibre-reinforced plastics material, the cylinder wall of which is provided with continuous printing openings which define an image which is to be printed.
A printing form of this nature is described, for example, in NL-A-8802144. The cylinder used in this printing form is made from plastics material, to which solid particles and/or fibres may be added in order to increase the mechanical stability and strength. The wall of the cylinder is provided with a large number of small perforations which are in the form of a grid of dots, i.e., printing openings, which define the image which is to be printed. The printing openings may be made with the aid of a laser.
U.S. Pat. No. 3,981,237 has likewise disclosed a printing form made from plastics for rotary screen printing, the cylinder wall of which may comprise one or more layers. This patent states that, by suitably selecting the plastics material, such as polyester, it is possible to obtain the mechanical properties which are demanded for use as a printing form. In this known printing form, the printing openings may likewise be made with the aid of a laser. The plastics material may be filled with a reinforcing material such as glass fibres. Such a rotary printing screen is manufactured from a tubular blank made by conventional extrusion methods, which is stretched biaxially afterwards. These filler materials are (short) chopped fibres, which are distributed randomly in the plastics material, even after biaxially stretching. Accordingly the volume concentration of the fibres is 40% at maximum. However, the strength and rigidity of a screen thus manufactured leaves much to be desired.
The abovementioned printing forms made from plastics material, in which printing openings which define the printed image are provided, are also known in the art as plastics xe2x80x9cgalvanoxe2x80x9d, even though there is actually no electrodeposition of metals involved in the production of these printing forms.
U.S. Pat. No. 4,959,260 discloses a mesh fabric useful for manufacturing a flat printing screen and consisting essentially of conjugate filaments. Each of the conjugate filaments has a sheath made from a material having a high adhesive property to an emulsion and resin used for preparing a stencil from a screen, and a core made from a material having high dimensional stability and elastic recovery property. Furthermore the mesh fabric should meet well defined mechanical requirements. In such a fabric or a screen produced therefrom no plastics matrix is present, wherein the filaments are incorporated.
Nickel stencils for rotary screen printing are difficult to handle and are fragile, in particular if the stencil (still) contains printing paste. The thickness of the nickel stencils, which is of decisive importance for the rigidity, is limited by the size of holes required and the mesh number, and can only be increased to a limited extent using the standard electroforming techniques. In the case of plastics materials in which perforations can be made with the aid of high-energy radiation, it is in principle possible to make more robust printing forms, which are therefore easier to handle.
However, these known printing forms made from plastics have not attained commercial success. One of the possible reasons for this is the low rigidity of plastics in general. Plastics are less rigid than nickel, which is the preferred material for conventional screen-printing forms, by a factor of approximately 100. A nickel stencil has a modulus of elasticity E of approximately 90 GPa; a cylinder provided with perforations which is made from polyester, which is one of the more rigid plastics, has a modulus of elasticity E of approximately 2 GPa. The elastic deformations which occur, inter alia, when the printing form is used for rotary screen printing, are dependent on the modulus of elasticity and also on the wall thickness of the cylinder. A PET galvano must have a thickness of approximately 0.89 mm in order to attain the same level of rigidity as a nickel stencil with a thickness of 0.1 mm. A large wall thickness of this nature has an adverse effect on the printing properties and the possibility of making printing openings with the aid of a laser. In addition, thick-walled printing forms of this nature are less easy or even altogether impossible to push together for transport and/or storage, as is customary with the thin-walled metal screen-printing forms, which can be transported and stored pushed together in the shape of a kidney. Furthermore, it is known that under load plastics are deformed more readily as a result of creep than metals, such as nickel.
As has been already been suggested above in accordance with NL-A-8802144, plastics can generally be made more rigid by the addition of fillers and/or fibres. However, this Dutch patent application does not provide any insight into how the intended increase in mechanical stability can be achieved.
The object of the present invention is to provide a printing form for rotary screen printing made from fibre-reinforced plastics material, in which the wall thickness of the cylinder is such that the printing properties and the possibility of making printing openings with the aid of a laser, as well as the possibility of transporting and/or storing the forms pushed together, are not adversely affected, or are scarcely adversely affected.
Another object of the invention is to provide a printing form for rotary screen printing made from fibre-reinforced plastics material, the mechanical properties of which are improved compared to the printing form made from (fibre-reinforced) plastics material described in the prior art.
According to the invention, the printing form for rotary screen printing of the type described above is characterized in that the cylinder comprises at least one layer having fibres with at least one oriented direction of the fibres. A single plastics layer with fibres incorporated therein which are oriented in one direction (also referred to below as unidirectional fibres) has anisotropic elastic properties, i.e., the properties are dependent on the direction in which the load is acting.
The addition of fibres to plastics generally imparts an improved rigidity. Fibres may be added in the form of so-called short fibres (also known as xe2x80x9cchopped fibresxe2x80x9d), as long fibres which are arbitrarily distributed in the plastics material, and as unidirectional fibres. The use of unidirectional fibres in principle provides the highest rigidity which can be achieved. Furthermore, the highest fibre content can be achieved with layers having unidirectional fibres, and consequently it is with these fibres that the highest modulus of elasticity can be attained.
As has already been stated, a single plastics layer having unidirectional fibres has anisotropic elastic properties. According to the invention, this anisotropy is used to limit the deformation in the direction of the load.
In this specification a fibre-reinforced plastics material is meant to be a plastics matrix layer, wherein reinforcing fibres are incorporated. According to the invention these fibres are oriented in one direction.
In addition to the customary use of the printing form according to the invention for rotary screen printing, in which printing ink or printing paste is applied to a substrate which is to be printed through the continuous printing openings, the printing form is also suitable for applying other coating materials to a substrate of this nature. One example of a coating material of this nature is a hot-melt adhesive. In other words, the expression xe2x80x9cfor rotary screen printingxe2x80x9d is not restricted to using printing ink or printing paste to print a substrate, but rather, in a broad sense, comprises the application of a material to a substrate through the continuous openings while the printing form is rotating. The printing screen according to the invention may be a conventional screen from which a stencil is prepared using standard photoresist techniques, so that only those openings in the screen remain open, which define the image to be printed, the so called xe2x80x9ccontinuous printing openingsxe2x80x9d, while the other openings in the screen are covered and/or filled by the cured portions of the photoresist. The printing screen according to the invention is preferably a galvano, comprising only continuous printing openings, which may have been made by a laser.
The invention also relates to a rotary screen-printing device provided with a printing form according to the invention.